For forming gobs of glass, for example intended to make hollow glass containers, it is known to form a column of molten glass, to convey the column in vertical direction and to cut the column itself transversely to form a sequence of gobs, which are then conveyed towards a respective forming mold.
Scissor devices are used for cutting the glass column, in which the glass column is crossed between a pair of blades moveable from and towards each other and towards an approached cutting position, in which they partially overlap causing the detachment of the gob.
Despite being used, such scissor devices are not very satisfactory because they do not allow to significantly increase the cutting speed or the production frequency and because, in all cases, they locally modify both the chemical-physical properties and the geometric features of the glass in the cutting zone, and consequently those of the formed gobs.
This is consequent to the fact that the cut is performed when the blades are arranged near a dead or motion reversion point, and thus during a step of deceleration of the blade themselves.
Consequently, the blades remain in contact with the glass for a relatively long time. Long contact times cause the localized cooling and thus the hardening of the glass because both blades are generally cooled by spraying a refrigerant liquid on each blade to prevent the sticking of the glass on the blades themselves.
The refrigerating liquid itself also strikes the glass column when the blade stroke is decreased in order to increase the production rate, and therefore the spraying members are close to the column. The hardening of the glass in the cutting zone is responsible for thermal-mechanical faults, typically known as “shear marks” which are found at the end on many finished products.
Additionally, during cutting, the blades transmit a flexural torque deriving from the partial overlapping during the step of cutting to the column; such a flexural torque changes the gob geometry in unexpected manner. Furthermore, the lower blade generates an imbalance of the newly cut gob which imposes the use of gob guiding surfaces.
Alternatively to the scissor devices, it is known, for example from patent application EP365177A1, to use single two-sided blade cutting devices. The blade is translated by means of reciprocating rectilinear motion and performs one cut during a delivery stroke and one cut during a return stroke.
Such devices allow to increase the cutting speed and the production rates because they reach the glass column when the translation speed is maximum, but the fact of using relatively thick blades, necessary to ensure cutting stability, do not solve the problem of flexural torque on the glass column. For this reason, such devices are always provided with gob guiding devices which intervene according to the direction of movement of the blade to define a rest or lateral reference for the glass.
In addition to the translating blades, it is known from U.S. Pat. No. 5,269,828 to use a disc blade, which rotates about a vertical axis parallel to the conveying direction of the column and has a recess delimited by the radial blade. Although such a solution allows to increase the cutting speed, it generates high thrusts on the column deriving from the fact that the blade is very thick, this being indispensable to guarantee the indeformability of the blade, the cutting edge of which must have a free peripheral stretch.
Furthermore, such a cutting device is not satisfactory if several columns must be cut at the same time. Indeed, in such conditions, the rotating blade cuts the gobs nearly simultaneously but at different speeds, because the columns are arranged at different distances from the rotation axis. The gobs thus made have deformations which are not only localized but are also different from one gob the next. For the same reasons, it is difficult to make efficient gob guiding devices.